The goal of this project is to develop a rubric that links the study of motor-unit fatigability in reduced-animal (anesthetized-cat) preparations with that in conscious humans. In addition, the goal is to link the study of motor-unit fatigability to that of the orderly motor-unit recruitment phenomenon and Hennaman's "Size Principle." The project is based upon two specific aims. The first examines the premise that the adaptive properties of motoneurons during both sustanied and brief repetitive current injections are associated with their functional thresholds and the force- producing capacity of the muscle fibers they supply. This aim will be tested in deeply anesthetized cats by measuring the biophysical properties of spinal motoneurons and the force output of their motor units and testing for an association between these properties and changes in mononeuronal firing rates brought about by intracellular injection of depolarizing currents of variable strength, duration, waveform and intermittency. The second specific aim tests the proposition that the fatigue of motor units during sustained and various forms of intermittent activation are associated with their normal recruitment order and their peak force-producing capacity. This second aim will be tested in animal preparations similar to the first aim, by measuring a variety of motor-unit properties in a hindlimb muscle, including the responses to newly developed fatigue-inducing stimulus regimens designed to stress the motor units in a relatively similar manner, irrespective of their contraction times, fusion frequencies and peak force-producing capacities. It is anticipated that these studies will contribute information on fundamental issues in motor control that have not yet been resolved. Furthermore, the work has profound implications for orthopedics, sports medicine, the fatigability that is evident in many neuromuscular diseases, motor recovery after brain damage and peripheral neuromuscular damage, and for the emerging field of motor prosthetics.